The present invention relates to the cloning of the gene expressing antigens of Actinobacillus pleuropneumonia (A. pleuropneumoniae). It further relates to a method of producing these antigens and the use of the antigens to vaccinate pigs against porcine pleuropneumonia.
Haemophilus pleuropneumonia of swine is a highly contagious respiratory disease caused by the gram-negative bacterium, A. pleuropneumoniae. In recent years, partly because of the trend toward confinement and intensified production, there has been a significant increase in the incidence of the disease and it is now a major cause of economic loss to the swine industry. During outbreaks of the acute disease the mortality rate can reach 100% among piglets and 25% among feeder pigs. Infected pigs may develop acute local extensive pneumonia accompanied by a fibrinous pleuritis or chronic localized pulmonary necrosis is with pleuritic adhesions. Eight serotypes of A. pleuropneumoniae have been identified but serotype 5 is by far the most prevalent.
It appears that one of the virulence factors of A. pleuropneumoniae is a secreted cytotoxin. This is supported by the fact cell-culture supernatants from A. pleuropneumoniae have been shown to be cytotoxic for porcine alveolar macrophages and peripheral monocytes (Bendixin et al., Infect. Immun. 33, 673-676 (1981)). Additionally, sonicated bacteria and sterile culture supernatants have been reported to induce localized pneumonia which is similar to pneumonia observed in naturally infected pigs (Rosendal et al., Proc. Int. Pig. Vet. Soc. Congr. 5: 221 (1980)).
It is believed that the A. pleuropneumoniae cytotoxin is an extracellular hemolysin/s produced by most if not all A. pleuropneumoniae serotypes. The nature of the hemolysin/s is poorly understood. It has been reported that the various serotypes of A. pleuropneumoniae produce either heat-stable carbohydrates (Kume et al., Infect. Immun. 51, 563-570 (1986)) or heat labile proteins (Maudsely et al., Can. J. Microbiol. 32, 801-805 (1986)). It has also been reported that the hemolysins of A. pleuropneumoniae serotypes 1, 2, 3, 5, 6,and 7 require RNA (Martin et al., Can. J. Microbiol. 31, 456-462 (1985)). To date, only two hemolysins have been characterized, a heat stable hemolysin from serotype 2 (Kume et al., Infect. Immun. 51, 563-570 (1986)) and a 105 KD polypeptide secreted by serotype 1 (Frey et al., Infect. Immun. 56, 2570-2575 (1988)). The amino acid sequence of any A. pleuropneumoniae hemolysin(s) has been unknown until the current invention.
There is currently no commercially available vaccine for porcine pleuropneumonia. Immunizations have been attempted using heat killed or formalin fixed bacteria but the efficiency of these immunogens has not been clinically proven. It is expected that the A. pleuropneumoniae hemolysin(s) can be used as a protective immunogen for pigs against porcine pleuropneumonia.